Proc. Nat. Acad. Sci. USA Vol. 71, No. 3, pp. 797-801, March 1974

Serotonin-Sensitive Adenylate Cyclase in Neural and Its Similarity to the : A Possible Site of Action of Lysergic Diethylamide (//) JAMES A. NATHANSON AND Department of , Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510 Communicated by Edward A. Adelberg, November 3, 1973 ABSTRACT An adenylate cyclase (EC 4.6.1.1) that is to potassium (B receptors) or to chloride (C receptors) (5, 6); activated specifically by' low concentrations of serotonin all three of these effects of serotonin in the mollusc are has been identified in homogenates of the thoracic ganglia of an - . The activation of this known to be blocked by D-lysergic acid diethylamide (LSD). by serotonin' was selectively inhibited by extremely low Serotonin can also decrease membrane ion permeability in concentrations of D-lysergic acid diethylamide (LSD), these same ganglia, an effect not blocked by LSD (7). In addi- 2-bromo-LSD, and , agents which are tion, serotonin appears to have direct excitatory effects on known to block certain serotonin receptors in vivo. The inhibition was competitive with respect to serotonin, and and (8, 9) and to stim- the calculated inhibitory constant of LSD for this sero- ulate fluid secretion in the insect salivary gland (10). tonin-sensitive adenylate cyclase was 5 nM. The data are In 1960, Mansour et al. (11) made the important observa- consistent with a model in which the serotonin receptor of tion of the existence, in cell-free preparations of whole neural tissue is intimately associated with a serotonin- flukes, of an adenylate cyclase activated either by low (10-v- sensitive adenylate' cyclase which mediates . The results are also compatible with 10-6 M) concentrations of serotonin or by somewhat higher the'possibility that 'some of the physiological'effects of (10-5-10-4 M) concentrations of LSD. (There was no report LSD may be mediated through interaction with serotonin- in that study of an antagonistic action of LSD on serotonin- sensitive adenylate'cyclase. stimulated adenylate cyclase activity.) In view of the large There is at present some evidence that the neuronal receptors amount of evidence suggesting a role for serotonin in neural for certain may be intimately associated function, itseemed of great importance toestablishtheexistence with the enzyme adenylate cyclase (EC 4.6.1.1), and that of serotonin-sensitive adenylate cyclase inneuraltissue. Despite the action of these neurotransmitters may be mediated the clear demonstration by Kakiuchi and Rall (12) that sero- through an increase in the intracellular level of tonin can increase cyclic AMP levels in slices of rabbit , 3':5'-monophosphate (cyclic AMP) in postsynaptic efforts in several laboratories, including our own, to demon- (1). In the mammalian , for example, a dopa- strate serotonin-sensitive adenylate cyclase in cell-free prepa- mine-sensitive adenylate cyclase has been identified (2) which rations of mammalian nervous tissue have not yet proven has properties quite similar to those of the caudate "dopamine successful. However, recently we were able to demonstrate receptor," characterized previously by a variety of biochem- the existence, in homogenates of insect nervous tissue, of an ical, pharmacological, and behavioral data in animals and in adenylate cyclase activated specifically by low concentrations man. In the , a norepinephrine-sensitive adenylate of serotonin (13). This serotonin-sensitive adenylate cyclase, cyclase may mediate the inhibitory effect of norepinephrine which was found in cockroach thoracic ganglia, was shown to be on Purkinje cells (3). distinct from two other adenylate cyclases (activated by Recent work suggests that the indoleamine, serotonin (5- dopamine and octopamine, respectively) found in the same hydroxytryptamine), may act as a at cer- tissue. tain in both vertebrate and invertebrate central We now report that this serotonin-sensitive adenylate cy- nervous systems. In the rat, for example, serotonin appears clase present in neural tissue has pharmacological properties to exert an inhibitory effect both on the serotonin-containing similar to those of known serotonin receptors and suggest that neurons of the , as well as on neurons receiving serotonin-sensitive adenylate cyclase may, in fact, mediate serotonergic connections from the raphe nuclei (4). In mollus- the physiological action of serotonin at some serotonergic can central ganglia, serotonin has an excitatory effect on synapses. Included among our evidence is the finding that certain neurons, mediated through a selective increase in so- the hallucinogen D-lysergic acid -diethylamide, which is dium permeability (A receptors), and inhibitory effects on thought to exert its behavioral effects through interaction other neurons, mediated through an increase in permeability with serotonin receptors, is capable, at extremely low concen- trations, of completely inhibiting the activation-by serotonin of serotonin-sensitive adenylate cyclase.

Abbreviations: LSD, -lysergic acid diethylamide; BOL, 2- METHODS bromo-LSD; EGTA, ethyleneglycol bis(,6-aminoethyl ether)- The effect of putative neurotransmitters on adenylate cyclase N,N'-tetraacetate. activity was measured as described previously (13). In our 797 Downloaded by guest on September 27, 2021 798 Biochemistry: Nathanson and Greengard Proc. Nat. Acad. Sci. USA 71 (1974) 225 r All experiments were replicated 2-4 . The results 200 shown in Figs. 1-5 represent typical experiments. RESULTS o 175 In invertebrate ganglia, LSD is known to block the effects of ". 0 ' serotonin on certain neurons which receive serotonergic input ! 150 A. (5, 6). In the present studyof insect ganglion homogenates, very low concentrations of LSD inhibited the stimulation of gangli- <,49 125 =i onic adenylate cyclase due to serotonin. In the experiment C) shown in Fig. 1, in the absence of LSD, 2.5 X 106 M serotonin *wI caused a stimulation of adenylate cyclase activity to 163% of the control value. Increasing concentrations of LSD pro- 75 L gressively inhibited the stimulation due to this concentration of serotonin. The stimulation due to serotonin was reduced 1010 108 lO 7 10-6 10-5 LSD. CONCENTRATION WM) by 50% in the presence of 2 X 10-8 M LSD and was almost FIG. 1. Effect of various concentrations of LSD on the activity abolished in the presence of 1 X 10-7 M LSD. of adenylate cyclase, in insect ganglion homogenate, in the pres- Concentrations of LSD greater than 1 X 106 M stimulated ence (broken line) or absence (solid line) of 2.5 X 10-6 M sero- enzyme activity, in the presence or absence of serotonin (Fig. tonin. Control activity in the absence of serotonin and LSD was 1). For example, 1.25 X 10-5 M LSD stimulated enzyme 10.8 1 1.2 pmol/mg of protein per min (mean 4 mean deviation activity to about 210% of the control value in the presence for four samples). In the absence of LSD, 2.5 X 10- M serotonin or absence of 2.5 X 106 M serotonin. Thus, depending upon stimulated enzyme activity to 163% of control. The values shown present, LSD had a dual effect on adenylate in Figs. 1-5 are the means and ranges for two to three identical the concentration samples, each assayed in duplicate. cyclase activity: at extremely low concentrations it inhibited the stimulation of activity due to serotonin, while at somewhat higher concentrations LSD, itself, stimulated the formation of earlier experiments we obtained evidence for distinct sero- cyclic AMP. tonin-, octopamine-, and dopamine-sensitive adenylate cy- The inhibitory effect of low doses of LSD was found to be clases in homogenates of cockroach ganglia. However, we specific for stimulation of adenylate cyclase activity by sero- were not able to obtain evidence for-a distinct norepinephrine- tonin. In the experiment shown in Fig. 2, in the absence of sensitive adenylate cyclase, and it seems probable that the LSD, 2.5 X 106 M dopamine caused a 1.85-fold stimulation, observed stimulatory effect of norepinephrine on adenylate 2.5 X 10-6 M octopamine, a 3.8-fold stimulation, and 5 X cyclase activity of this preparation is attributable to a partial 10-5 M norepinephrine, a 3.7-fold stimulation of adenylate activation by norepinephrine of octopamine- and dopamine- 450 sensitive adenylate cyclases. + OCTOPAMINE Briefly, pro-, meso-, and metathoracic ganglia from adult -.- 400 male cockroaches, Periplaneta americana, were homogenized L ..- 4 (20 mg/ml) in 6 mM - maleate buffer (pH 7.4) containing 4P___- ,__*--_ U + NOREPINEPHRINE 350 _ 2 mM EGTA [ethyleneglycol bis(P-aminoethyl ether)-N,N'- o, c tetraacetate]. Adenylate cyclase activity was measured by 0 200~ 300 O 0 0 determining the rate of formation of cyclic AMP from ATP 0 in an assay system containing (in mmol/liter): Tris * maleate F/. --_ = 175 -I_I-- 250 a. (pH 7.4), 80; , 10; MgSO4, 2; EGTA, 0.5; ATP, 2 49 wet 00 0.5; and tissue homogenate (0.5 mg of weight of cockroach 200 2 ganglia), plus putative neurotransmitter or other test sub- stances as indicated, in a final volume of 200 Al. Incubation 0 in a water The reaction was 25 150 was for 3 min at 30° shaken bath. 4c initiated by the addition of ATP, and terminated by boiling U .j 100 -______4-- __- 100 for 30 sec. The reaction mixture was then centrifuged at low U to remove insoluble material. Cyclic AMP in the super- U speed 75 L 50 natant was measured by the method of Brown et al. (14). I4-I Under the experimental conditions used, enzyme activity was 0 10-9 10-s 10-T 10-6 linear with respect to and enzyme concentration. In all LSD CONCENTRATION CM) experiments the adenylate cyclase activity of homogenates, FIG. 2. Effect of various concentrations of LSD on the activity measured in the presence of added test substance(s), was ex- of adenylate cyclase, in insect ganglion homogenate, in the control measured in the presence of either 2.5 X 10- M dopamine, 2.5 X 10-6 M octo- pressed as a percentage of activity For the of test substance. pamine, or 5 X 10- M norepinephrine. comparison, absence added the effect of LSD on in the pres- were data showing enzyme activity LSD and 2-bromo-D-lysergic acid diethylamide (BOL) ence of 2.5 X 106 M serotonin are reproduced from Fig. 1. obtained from the Food and Drug Administration-National Stimulation due to octopamine or norepinephrine is shown on the Institute of Mental Health. Cyproheptadine [4-(5H-dibenzo- right-hand ordinate; stimulation due to serotonin or dopamine is [a,d]cyclohepten-5-ylidene)-1-methylpiperidine] was sup- shown on the left-hand ordinate. Control activity in the absence plied through the courtesy of Dr. Clement Stone, Merck and of added compound was 12.0 ± 1.6 pmol/mg of protein per min Co., Inc. (mean ± mean deviation for four samples). Downloaded by guest on September 27, 2021 Proc. Nat. Acad. Sci. USA 71 (1974) Serotonin-Sensitive Adenylate Cyclase and LSD 799

200 r 250

00 175 SEROTONIN + 225 0#1. I- 0 C 0 = 200 -o 150 I _ 00 O- 0C 0. ° 175 F 2 125 0 4 IF 2 Cf 150

0 4c 125 -J 75 L U af~%100 _- 0 1o-6 lo-7 10-6 lo-, CONCENTRATION OF SOL OR CYPROHEPTADINE CM) 75 FIG. 3. Effect of various concentrations of the serotonin p antagonists, BOL or cyproheptadine, on the activity of adenylate 50 cyclase, in insect ganglion homogenate, in the presence (solid lines) or absence (broken lines) of 2.5 X 10-6 M serotonin. 10 7 10 6 10 ° 10 4 10 3 Control activity in the absence of serotonin, BOL, and cypro- CONCENTRATION OF TEST SUBSTANCE CM) heptadine, was 13.3 4 1.0 pmol/mg of protein per min (mean FIG. 4. Effect of various concentrations of serotonin, LSD, mean deviation for four samples). BOL, or cyproheptadine on adenylate cyclase activity in insect ganglion homogenate. Control activity in the absence of added cyclase activity, compared with that of control. Concentra- compound was 10.0 4 1.5 pmol/mg of protein per min (mean 4 tions of LSD which inhibited the stimulation of adenylate mean deviation for four samples). cyclase activity by- serotonin had little or no effect on the stimulation of adenylate cyclase activity by octopamine, Even in the of a dopamine, or norepinephrine. For example, in the presence activity. presence serotonin, depression of of 1 X 10-7 M LSD, a concentration which virtually abolished enzyme activity below control levels was seen in the presence the stimulation of adenylate cyclase activity by serotonin, of high concentrations of cyproheptadine (Fig. 3). The effect of a fixed concentration or there was little or no inhibition of dopamine-, octopamine-, of LSD, BOL, cy- or norepinephrine-stimulated activity. proheptadine was tested on adenylate cyclase activity in the The LSD derivative, 2-bromo-D-lysergic acid diethylamide presence of various concentrations of serotonin. Fig. 5 shows (BOL), and the structurally unrelated compound, cyprohep- that LSD and BOL each caused a shift in the serotonin dose- tadine, are known to block serotonin receptors in both verte- response curve to the right, resulting in an increase in the brate and invertebrate tissues (8, 9, 15). Fig. 3 shows that activation constant, Ka (the concentration required to cause these two serotonin antagonists, like LSD, inhibited the stim- a half-maximal increase in enzyme activity), for serotonin, ulation, by serotonin, of adenylate cyclase activity in insect without causing any significant change in the maximal activa- ganglion homogenates. A 50% inhibition of the stimulation tion of the enzyme (Vm.s). A similar shift in the dose response curve in due to 2.5 X 10- M serotonin was obtained with either 1 X was seen the presence of cyproheptadine (data not 10-8 M BOL or 2 X 10-7 M cyproheptadine. In other experi- shown). This type of interaction between an and an ments it was found that the inhibitory effects of both BOL and cyproheptadine on enzyme activity were relatively TABLE 1. Inhibition by various antagonists of the specific for serotonin stimulation: both BOL and cyprohepta- stimulation of adenylate cyclase activity by various putative dine were considerably less effective in inhibiting stimulation neurotransmitters of adenylate cyclase activity due to dopamine, octopamine, or norepinephrine (see below and Table 1). Stimula- Approximate Ki for Like LSD, BOL had a direct stimulatory effect on adenylate tion of cyclase activity, when used alone at concentrations of 1 X Cypro- enzyme LSD BOL heptadine Phent 10- M or more (Figs. 3 and 4). (Unexpectedly, the combina- by (nM) (nM) (Mm) (MM) tion of 2.5 X 106 M serotonin plus 1 X 106 M BOL caused less stimulation of enzyme activity than did either substance 5-HT 5 5 0.25 3 alone.) Fig. 4 compares the activation of adenylate cyclase DA >10,000 100 1.0 2 OCT >10,000 1.0 0.5 activity by either LSD (Ka > 4 X 10- M), BOL = 1.5 X >10,000 (K, NE >10,000 >10,000 1.0 0.6 10- M), or serotonin (K. = 6 X 10-7 M). (Because of the small amounts available to us, it was not practical to use con- The Ki values represent the calculated constants centrations of LSD greater than 1 X 10-4 M.) It should be inhibitory emphasized that the stimulatory effect of BOL, like that of of the serotonin antagonists, LSD, BOL, and cyproheptadine, as well as of the alpha- antagonist, LSD, was obtained at concentrations much greater than the (Phent), for adenylate cyclase activity of cockroach ganglion concentrations required to inhibit the activation of the enzyme homogenates, stimulated by either serotonin (5-HT), dopamine by serotonin. In contrast to thestimulation of adenylate cyclase (DA), octopamine (OCT), or norepinephrine (NE). The data activity by LSD, BOL, or serotonin, cyproheptadine, at similar are derived from experiments like those shown in Figs. 1-3 and concentrations, caused a depression of basal adenylate cyclase Fig. 5. Further details are in the text. Downloaded by guest on September 27, 2021 800 Biochemistry: and Nathanson Greengard Proc. Nat. Acad. Sci. USA 71 (1974)

200 C 0.02 A CONTROL

*b 175 -°0.1 01 0.3 0.5 '/Cs3 (AlM)

0 150 -x 0. ~~~~~~~~~-+BOL 125 -+LSD /(2.5X1O,7M)

o~ ~ ~ o m

>- 100 - - -

75 17I0- Io-5 1o-4 10-3 SEROTONIN CONCENTRATION CM) FIG. 5. Effect of various concentrations of serotonin on the activity of adenylate cyclase, in insect ganglion homogenate, in the absence of inhibitor (control) and in the presence of LSD (1 X 10-7 M) or BOL (2.5 X 10-7 M). Basal enzyme activity in the absence of added compound was 15.8 4 2.0 pmol/mg of protein per min (mean 4 mean deviation for four samples). Inset: double reciprocal plot of the percent increase in enzyme activity due to serotonin, as a function of the serotonin concentration from 2 X 106 M to 1.5 X 10-4 M. A: control;B:1 X 10-7MLSD;C:2.5 X 10-7MBOL. inhibitor indicates inhibition of a competitive type. A double Table 1 also includes the results of studies of the effect of reciprocal plot of the data (inset in Fig. 5) also illustrates the the alpha-, phentolamine, on the neuro- competitive nature of the inhibition, by LSD or BOL, of the transmitter-sensitive adenylate cyclase activity of insect serotonin-induced increase in enzyme activity. ganglion homogenates. Phentolamine competitively inhibited In other experiments it was found that stimulation of the stimulation of adenylate cyclase activity caused by sero- adenylate cyclase activity by dopamine, octopamine, or tonin, dopamine, octopamine, or norepinephrine. In contrast norepinephrine, could be inhibited to some extent by suffi- to the serotonin antagonists, phentolamine was not more ciently high concentrations of any of the serotonin antagonists, effective in inhibiting serotonin-sensitive adenylate cyclase LSD, BOL, or cyproheptadine. The inhibition appeared in all activity (K1, 3 X 10- M) than in inhibiting dopamine- instances to be of a competitive nature. Using competitive sensitive (Ki, 2 X 10- M), octopamine-sensitive (Ki, 5 X kinetics, the inhibitory constants (Ki values) of LSD, BOL, 10-7 M), and norepinephrine-sensitive (Ki, 6 X 10-7 M) and cyproheptadine for serotonin-sensitive adenylate cyclase enzyme activity. activity, as well as for octopamine-, dopamine-, and norepi- nephrine-stimulated activity, were calculated from the ex- DISCUSSION perimental data. The inhibitory constants shown in Table 1 The present results demonstrate the presence of a serotonin- were calculated from data of the type presented in Fig. 5, in sensitive adenylate cyclase in invertebrate neural tissue, and which the inhibitor concentration was held constant and show that activation of this enzyme is selectively inhibited by the concentration of putative neurotransmitter was varied, as agents which are known to block certain serotonin receptors well as from data of the type shown in Figs. 1-3, in which the in vivo. Thus, LSD and BOL (and cyproheptadine to a lesser concentration of putative neurotransmitter was held constant, extent) were far more effective in blocking stimulation due to and the inhibitor concentration was varied. The two types serotonin than they were in blocking stimulation due to of experiments gave similar results. The apparent Kj of LSD dopamine, norepinephrine, or octopamine. Conversely, and of BOL for the serotonin-sensitive adenylate cyclase was phentolamine, an alpha-adrenergic antagonist, was not more approximately 5 X 10-9 M. (The apparent Ki of LSD for the effective in inhibiting stimulation due to serotonin than in serotonin-sensitive adenylate cyclase was independent of the inhibiting stimulation due to the other putative neurotrans- concentration of ATP present.) Both LSD and BOL were mitters. quite ineffective in inhibiting octopamine- and norepi- Extremely low concentrations of both LSD and BOL were nephrine-stimulated adenylate cyclase activity (Ki > 1 X 10-5 effective in inhibiting serotonin-stimulated enzyme activity. M). LSD was also quite ineffective in inhibiting dopamine- The calculated inhibitory constants for these two serotonin sensitive adenylate cyclase activity (Kj > 1 X 10-5 M), antagonists were 5 X 10-9 M, and that for cyproheptadine was whereas BOL was somewhat more effective (K1, 1 X 10-v M). 2.5 X 10-7 M. The fact that LSD inhibits serotonin-sensitive The inhibition of serotonin-sensitive adenylate cyclase by adenylate cyclase activity at such low concentrations is of cyproheptadine was much less specific than that shown by particular interest in view of the great potency of LSD as a LSD: the Ki of cyproheptadine for serotonin-sensitive hallucinogenic agent in man: the K1 for LSD (5 X 10-9 M) adenylate cyclase was 2.5 X 10-7 M, whereas the Ki of cypro- is similar to the concentration (2.5 X 10-9 M) estimated by heptadine for dopamine-, octopamine-, and norepinephrine- Axelrod et al. (16) to be present in brain when a sensitive adenylate cyclase was about 1 X 10- M, i.e., only hallucinogenic dose (1 Ag/kg) of LSD (17) is administered. about 4 times greater. Although, at low concentrations, LSD and BOL inhibited Downloaded by guest on September 27, 2021 Proc. Nat. Acad. Sci. USA 71 (1974) Serotonin-Sensitive Adenylate Cyclase and LSD 801

serotonin-sensitive adenylate cyclase, at higher concentra- In the present experiments LSD had a dual effect on tions (1 X 10-lIM or greater) both LSD and BOL directly adenylate cyclase activity (inhibition of serotonin-stimulated stimulated adenylate cyclase activity. This concentration- activity at low concentrations but direct stimulation of en- dependent dual effect of LSD and BOL is of interest, since, in zyme activity at higher concentrations). It is possible that physiological studies, LSD has been variously reported to act in the mammalian brain LSD might either stimulate adenyl- as either a serotonin agonist or antagonist depending upon the ate cyclase activity directly or inhibit the stimulation of sero- tissue studied and the concentration of LSD used (4, 6, 8, 9, tonin-sensitive adenylate cyclase activity by serotonin. In 18-20). It has also been reported (21) that synaptic mem- any case, because of the present evidence that LSD affects brane fractions of rat contain separate low serotonin-sensitive adenylate cyclase activity of invertebrate affinity and high affinity binding sites for LSD. Interestingly, nervous tissue at very low concentrations, it will be important the reported dissociation constant for the high affinity binding to examine extracts of the raphe nuclei, as well as of other sites (9 X 10-9 M) and that for the- low affinity binding sites selected parts of the mammalian brain, for serotonin-sensitive (1.2 X 10-6 M) are similar, respectively, to the inhibitory adenylate cyclase activity, and to determine whether the constant of LSD for serotonin-sensitive adenylate cyclase (5 effects of LSD, BOL, and other serotonin antagonists on this X 10- M) and the activation constant of LSD for adenylate enzyme can provide an explanation, at the molecular level, cyclase (slightly greater than 4 X 10- M) found in the pres- for the effects of LSD in man. ent experiments. Current evidence suggests that serotonin may be a neuro- This work was supported by USPHS Grant NS-08440 and transmitter in invertebrate ganglia (5-7, 22). Using intact MH-17387. cell preparations, Cedar and Schwartz (22) demonstrated that serotonin raises cyclic AMP levels in isolated Aplysia 1. Greengard, P., Nathanson, J. A. & Kebabian, J. W. (1973) ganglia. However, because they could not demonstrate an Biochem. Pharmacol., in press. Also in Frontiers in - effect of serotonin in homogenates, these authors were not able Research, eds. Usdin, E. & Snyder, S. (Pergamon to show that serotonin directly affected adenylate cyclase ac- Press, New York and London), in press. tivity. The results of the present study, which demonstrate 2. Kebabian, J. W., Petzold, G. L. & Greengard, P. (1972) Proc. Nat. Acad. Sci. USA 69, 2145-2149. the presence in insect ganglia of an adenylate cyclase which is 3. Hoffer, B. J., Siggins, G. R., Oliver, A. P. & Bloom, F. E. selectively activated by serotonin and specifically inhibited (1972) Advan. Cyclic Nucleotide Res. 1, 411-423. by low concentrations of agents which are known to block 4. Haigler, H. J. & Aghajanian, G. K. (1974) J. Pharm. Exp. invertebrate serotonin receptors, support the possibility that Ther., in press. serotonin-sensitive cyclase 5. Cottrell, G. A. (1970) Nature 225, 1060-1062. adenylate may mediate serotoner- 6. Gerschenfeld, H. M. (1971)Science 171, 1252-1254. gic neurotransmission in invertebrate ganglia. It will be im- 7. Paupardin-Tritsch, D. & Gerschenfeld, H. (1973) Nature portant, in future studies, to determine the electrophysiologi- New Biol. 244, 171-173. cal effects of cyclic AMP derivatives on vertebrate and in- 8. Cerletti, A. & Doepfner, W. (1958) J. Pharm. Exp. Ther. vertebrate nerve cells which are known to contain serotonin 122, 124-136. 9. Beernink, K. D., Nelson, S. D. & Mansour, T. E. (1963) receptors. Int. J. Neuropharmacol. 2, 105-112. The pharmacological properties of the serotonin-sensitive 10. Berridge, M. J. & Prince, W. T. (1972) Advan. Cyclic adenylate cyclase studied in the present investigation are NuckotideRes. 1, 137-147. similar to those of serotonin receptors in the rat uterus and A- 11. Mansour, T. E., Sutherland, E. W., Rall, T. W. & Bueding, receptors in molluscan ganglia 8). In both of the latter E. (1960) J. Biol. Chem. 235, 466-470. (6, 12. Kakiuchi, S. & Rall, T. W. (1968) Mol. Pharmacol. 4, 367- cases, as well as in the present study, LSD and BOL antago- 378. nized the actions of serotonin and were approximately equi- 13. Nathanson, J. A. & Greengard, P. (1973) Science 180, 308- potent in doing so. In man, however, LSD is known to be 310. much more potent than BOL as a hallucinogenic agent (17). 14. Brown, B. L., Elkins, R. P. & Albano, J. D. M. (1972) Advan. Cyclic Nucleotide Res. 2, 25-40. Therefore, if LSD exerts its behavioral effects through inter- 15. Stone, C. A., Wenger, H. C., Ludden, C. T., Stavorski, actions with serotonin receptors, then one must postulate J. M. & Ross, C. A. (1961) J. Pharmac. Exp. Ther. 131, that the serotonin receptors in various tissues have different 73-84. characteristics. There is, in fact, some evidence which sug- 16. Axelrod, J., Brady, R. O., Witkop, B. & Evarts, E. V. (1957) gests that serotonin receptors in various tissues do differ in Ann. N.Y. Acad. Sci. 66, 435-444. 17. Isbell, H., Miner, E. J. & Logan, C. R. (1959) Psychophar- their characteristics. In the rat, for example, LSD blocks the macologia 1, 20-28. effects of serotonin on uterine muscle (8) but mimics the 18. Shaw, E. &Woolley, D. W. (1956)Science 124, 121-122. effects of serotonin on neurons of the raphe nucleus (4). Fur- 19. Costa, E. (1956) Proc. Soc. Exp. Biol. Med. 91, 39-41. thermore, BOL, which is as effective a serotonin blocker as 20. Berridge, M. J. & Prince, W. T. (1973) Nature New Biology LSD in the rat uterus (8), and gastropod mollusc (6), had 243, 283-284. 21. Farrow, J. T. & Van Vunakis, H. (1972) Nature 237, 164- considerably less effect than LSD in directly inhibiting raphe 166. discharge (4) and has effects opposite to those of LSD 22. Cedar, H. & Schwartz, J. H. (1972) J. Gen. Physiol. 60, in the liver fluke (9). 570-587. Downloaded by guest on September 27, 2021